The invention relates generally to communication systems. In particular, the invention relates to the dynamic reformatting of a time division multiple access frame dependent upon the demand of the attached stations.
Many modern, large-scale communication systems rely upon geosynchronous satellites acting as transponders between the transmitting and receiving stations. Although originally used for point-to-point communication between two ground stations, more recent satellite communication systems link together a substantial number of ground stations, offering selective communication between any pair of the ground stations. Such a system is schematically illustrated in FIG. 1 for N ground stations 10 linked together by a communication satellite 12 in geosynchronous orbit. The illustrated system is designed for telephone communications with each station 10 being associated with a telephone regional office. Whenever a telephone connection is desired between two telephone lines connected to different regional offices, the call is routed through the associated ground station 10 and is transmitted from there, through the satellite 12, to the appropriate receiving ground station 10.
Older satellite communication systems relied upon frequency allocation between the transmitting ground stations 10. However, more recent multi-point systems, particularly those designed to support telephone/data communications, have adopted a TDMA (time division multiple access) approach. Such a system is disclosed by Maillet in U.S. Pat. No. 3,649,764. In a TDMA system, data is not transmitted continuously but is time multiplexed. The transmission is divided into time frames 14 and 16 with each frame being further subdivided, according to a predetermined format, into traffic bursts TB. Both data and voice signals are transmitted in digital form. The frames repeat often enough that a telephone conversation can be made to appear continuous and instantaneous. In the illustrated example, each ground station 10 is assigned one traffic burst. The transmission of the traffic bursts from the individual ground stations 10 are synchronized so that they arrive at the satellite 12 in the proper time sequence to form the up-link frame 14. The communication satellite 12 receives the up-link frame 14 and retransmits the frame as the down-link framep 16. Although the satellite 12 amplifies and frequency shifts the up-link frame 14 into the downlink frame 16 and perhaps uses part of the frame for housekeeping purposes, the satellite 12 can be viewed as a passive transponder with the up-link frame 14 being identical to the down-link frame 16. It is of course to be appreciated that the frames 14 and 16 illustrated in FIG. 1 are only one pair of a nearly continuous series of up-link frames and down-link frames, the frames in each series being separated by the minimum necessary time.
The entire down-link frame 16 is received by each of the N stations 10 so tht each station 10 is receiving the transmissions of every other station 10. The individual traffic burst TB must contain additional information indicating for which of the ground stations 10 the transmission is intended.
In a TDMA system, a reference station 18 is usually present to provide some degree of coordination between the ground stations 10. At a minimum, the reference station 18 must synchronize the ground stations 10 so that the frames 14 and 16 are synchronized between the stations 10 and furthermore it synchronizes the traffic bursts TB within the frame.
One of the difficulties of a telephone-based communication system is the fluctuation in the loads of the various ground stations 10. These fluctions may be either statistical or predictable. A statistical fluctuation arises because the ground stations 10 has no control on the number of requests for a telephone connection and this number statistically varies with time. A predictable fluctuation would arise from different times of day for ground stations 10 located in different time zones. Nonetheless, for a consumer-based telephone/data system, there must be a high probability that, when a connection is demanded, channel capacity is available. If the frame format is fixed, this requirement for availability means that there must be a large amount of excess capacity within each of the traffic bursts TB. This in turn implies a relatively high bandwidth system.
Bandwidth is both scarce and, in the case of the satellite 12, expensive to support because of the correspondingly increased power level. Alternatively, for a fixed bandwidth, the excess capacity required for a high availablity with a fixed format implies a decreased number of reliably available channels.
In view of the problems of a fixed allocation between the multiple ground stations 10, demand assigned multiple access (DAMA) has been developed. By DAMA is meant that the allocation of time or bandwidth between the ground stations 10 is dynamically allocated according to a real-time demand for channel capacity demanded by the individual ground stations 10. Demand assigned multiple access has been traditionally used in single channel per carrier satellite communication systems, that is, frequency division rather than the time division illustrated in FIG. 1. Examples of these systems include the SPADE system, which has been implemented in the INTELSAT network. In the SPADE system, each earth station 10 communicates with all other stations 10 via a wide band common signalling channel. All call requests are communicated via this channel among all the stations 10 in the network. The different carrier channels, corresponding to different frequencies, are allocated to the different ground stations 10, according to these requests. Each station 10 maintains a data base that represents the frequency assignments for all carrier frequencies in the transponder of a satellite 12. The SPADE system represents a decentralized approach to channel allocation.
Other satellite systems have been designed for centralized control of single channel per carrier satellite communication networks. For example, a master control computer located in a reference station 18 polls each of the earth stations 10 in the network for call requests and thereafter assigns satellite frequencies as required to set up the desired calls. Both of the described DAMA systems have been used with frequency division rather than time division communication. However, demand assignment for a TDMA system is described by Edstrom in U.S. Pat. No. 3,848,093. It is not felt that either the centralized or the decentralized approaches are totally appropriate for a TDMA system. A totally decentralized system does not make efficient use of the channel capacity, assuming that there must be a high probability for completing a call request. A totally centralized system such as that of Yeh in U.S. Pat. No. 4,204,093, or of Rothauser et al. in U.S. Pat. No. 4,096,355, although efficient in call channel capacity, introduces excessive complexity and delays caused by the rapidly changing system configuration. Torng in U.S. Pat. No. 4,383,315 and Fennel, Jr. et al in U.S. Pat. No. 4,322,845 disclose a mixture of centralized and decentralized control. These problems with totally centralized or decentralized control will now be explained.
In a demand assigned TDMA network, the process of establishing a communication link between earth stations 10 requires the originating earth station to process the incoming call request from the telephone lines to determine the destination for this call. This call processing will result in a request for a portion of the TDMA frame in which to carry the traffic associated with the call, whether it be for voice or data communication. If a full duplex connection is required, as is the case for a typical voice call, then two requests will be generated. For a typical satellite transponder, between two and four call requests per second can be expected. Larger systems are designed with multiple transponders so that multiple frames are being received simultaneously. The allocation, or management of the TDMA frame, can be either centralized at the reference station 18 or decentralized among the ground stations 10.
A full evaluation of the benefits and disadvantages of the two approaches requires the introduction of some com unication terminology. Grade of service (GOS) is the probability that a call request cannot be honored by a station 10 because no space can be allocated to it. Obviously in a consumer market, the overall GOS should be minimized to prevent the undue occurrence of busy signals. As the number of calls approaches the number of available channels, the grade of service deteriorates, that is, GOS increases. A typical relation between the percentage usage of the channels and the grade of service is shown in FIG. 2, presented solely for illustrative purposes. Such curves vary depending upon system design. For an economically efficient system, the number of calls should approach the number of channels. However, this increased efficiency inevitably degrades the grade of service. On the other hand, a low value for the grade of service is desirable for high quality service, but it is economically expensive. An erlang is another measure of channel usage, particularly appropriate for TDMA systems. An erlang is the number of call-seconds per second for the system as whole. Obviously, a higher number of erlangs implies an efficiently used system. Because there are multiple channels handling multiple calls in a TDMA system, a TDMA system typically has an erlang value greater than one.
If the frame management functions for a TDMA system are centralized at the reference station 18, then the resultant system efficiently uses the available capacity. For instance, for a TDMA network having a raw capcity of 465 full duplex circuits, a fully centralized network can support 425 erlangs of traffic with a grade of service GOS=0.01. Although these parameters are impressive, such a system nonetheless has several drawbacks. The 2-4 call requests per second will require a very large computer at the reference station 18. The call requests all pass through the communication satellite 12 located approximately 36,000 miles above the ground stations 10 and the reference station 18. As a result, the delays associated with the propagation of the request to the reference station 18 and of the reply to the requesting station 10 can become appreciable, approaching 1 second. Each ground station 10 must reconfigure its timing controls to conform to a reconfigured TDMA frame. If this reconfiguration is occuring at the rate of 2-4 times a second, the frame management processing at each of the ground stations 10 becomes appreciable and additional channel capacity must be provided for the frequent call requests and resultant reconfiguration data. It is to be remembered that in a frequency division system, the frequencies are individually allocated so that the reallocation of one frequency does not require a complete reallocation of all the frequencies.
If, on the other hand, frame management were totally decentralized, each earth station 10 would have one segment or traffic burst of the TDMA frame for which it had the management responsibility. With this approach, the total network traffic capacity would be a function of the number of stations 10 in the network since each station must maintain a separate reserve capacity to satisfy the required grade of service. If the previously described TDMA network of 465 circuits was required to maintain the same grade of service among 30 stations 10, the fully decentralized approach would support 320 erlangs of full duplex traffic, a reduction from the 425 erlangs of the totally centralized approach. However, if the number of stations is increased to 100, the maximum full duplex traffic that could be supported falls further to 166 erlangs. Thus, system delays and complexity are reduced in the fully decentralized TDMA system but only at the expense of a significantly reduced traffic capacity.